Decreasing water supplies throughout much of the industrialized world necessitate new methods and systems for utilizing water including contaminants or impurities. Additionally, certain industries have a need for safer, more energy efficient methods and systems for removing water from a target material or solute. Traditional methods and systems of liquid treatment include thermal flash evaporation and membrane filtration. The most popular membrane filtration method is reverse osmosis, in which water is separated from solutes (e.g., contaminants) in a feed liquid by application of a pressure overcoming the osmotic pressure of the feed liquid. For water desalination processes, the pressure to overcome the osmotic pressure of the feed liquid can be substantial, such as greater than 50 atm, resulting in significant equipment and energy costs. Additionally, the application of pressure often exacerbates reverse osmosis membrane fouling by inorganic and organic molecules.
Forward osmosis circumvents several of the deficiencies of reverse osmosis by using osmotic pressure gradients across a semi-permeable membrane to diffuse water from the feed liquid into a draw solution. The enabling feature of the draw solution is a draw solute that enables the draw solution to have a greater osmotic pressure than the feed liquid. Conventional draw solutes include sulfur dioxide, aluminum sulfate, sugars (e.g., glucose, fructose, sucrose), potassium nitrate, ammonium carbonate, and ammonium carbamate. Disadvantageously, such draw solutes either require energy intensive processes, such as reverse osmosis, in order to be removed from the water they draw, or produce materials (e.g., SO2, gaseous ammonia) that are toxic, volatile, and correspondingly expensive to handle.
It would, therefore, be desirable to have a method and system of liquid treatment utilizing an effective, relatively less toxic, and relatively less volatile draw solute that is easily removed following forward osmosis. Such a method and system would reduce the energy demands and operational costs associated with conventional liquid treatment technologies, favorably affecting a broad array of operations such as bulk water purification (water desalination), produced water (e.g., waters brought to the surface during oil and gas drilling) utilization, solution mining (e.g., mining of underground, water-soluble minerals), carbon dioxide scrubbing, and acid gas scrubbing.